MNRAS 000,1–11 (2020) Preprint 18 November 2020 Compiled using MNRAS LATEX style file v3.0 LOFAR Detection of a Low-Power Radio Halo in the Galaxy Cluster Abell 990 N. D. Hoang ,1¢ T. W. Shimwell,2,3 E. Osinga,3 A. Bonafede,4 M. Brüggen,1 A. Botteon3, G. Brunetti,5 R. Cassano5, V. Cuciti 1 A. Drabent6, C. Jones7, H. J. A. Röttgering,3 and R. J. van Weeren3 1Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany 2Netherlands Institute for Radio Astronomy (ASTRON), P.O. Box 2, 7990 AA Dwingeloo, The Netherlands 3Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, the Netherlands 4Department of physics and astronomy, Bologna University - Via Zamboni, 33, 40126 Bologna, Italy 5IRA INAF, via P. Gobetti 101 40129 Bologna, Italy 6Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany 7Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA Accepted 2020 November 13. Received 2020 October 15; in original form 2020 May 18 ABSTRACT Radio halos are extended (∼ Mpc), steep-spectrum sources found in the central region of dynamically disturbed clusters of galaxies. Only a handful of radio halos have been reported 14 to reside in galaxy clusters with a mass "500 . 5 × 10 " . In this paper we present a LOFAR 144 MHz detection of a radio halo in the galaxy cluster Abell 990 with a mass of 14 "500 = ¹4.9 ± 0.3º × 10 " . The halo has a projected size of ∼700 kpc and a flux density of 20.2 ± 2.2 mJy or a radio power of 1.2 ± 0.1 × 1024 W Hz−1 at the cluster redshift (I = 0.144) which makes it one of the two halos with the lowest radio power detected to date. Our analysis of the emission from the cluster with Chandra archival data using dynamical indicators shows that the cluster is not undergoing a major merger but is a slightly disturbed system with a mean 44 −1 temperature of 5 keV. The low X-ray luminosity of !- = ¹3.66 ± 0.08º × 10 ergs s in the 0.1–2.4 keV band implies that the cluster is one of the least luminous systems known to host a radio halo. Our detection of the radio halo in Abell 990 opens the possibility of detecting many more halos in poorly-explored less-massive clusters with low-frequency telescopes such as LOFAR, MWA (Phase II) and uGMRT. Key words: galaxies: clusters: individual (Abell 990) – galaxies: clusters: intra-cluster medium – large-scale structure of Universe – radiation mechanisms: non-thermal – diffuse radiation 1 INTRODUCTION 1.4 GHz), steep-spectrum1 (U . −1) sources that pervade large regions of the cluster. They are apparently unpolarised down to a Diffuse (∼Mpc) synchrotron radio emission (i.e. halos) in the centre arXiv:2011.08189v1 [astro-ph.HE] 16 Nov 2020 few percent above ∼1 GHz. In turbulent re-acceleration model (e.g. of galaxy clusters indicates the presence of relativistic particles and Brunetti et al. 2001; Petrosian 2001; Brunetti & Lazarian 2007), large-scale magnetic fields in the intra-cluster medium (ICM). It is radio halos are generated by merger-induced turbulence that ac- well known that the acceleration of particles to relativistic energy celerates cosmic ray (CR) particles and/or amplifies the magnetic and the amplification of magnetic fields at these scales are associated fields in the ICM. In addition, hadronic models that attempt to ex- with dynamical events in the host cluster of galaxies. However, the plain the generation of relativistic particles by the inelastic collisions physical mechanisms governing the particle acceleration and the between relativistic protons and thermal ions in the ICM seems to magnetic field amplification have not been fully understood (e.g., play a lesser role in the formation of radio halos (Ackermann et al. Brunetti & Jones 2014; van Weeren et al. 2019). 2010; Brunetti & Jones 2014; Ackermann et al. 2016; Brunetti et al. Radio halos are extended (∼Mpc), faint (∼μJy arcsec−2 at 2017), but could be a more significant process in mini-halos that ¢ E-mail: [email protected] 1 the convention ( / a U is used in this paper © 2020 The Authors 2 N. D. Hoang et al. have smaller sizes (. 500 kpc; Pfrommer & Enßlin 2004; Zandanel covering 27 percent of the northern sky will be released in the et al. 2014; Brunetti & Jones 2014). upcoming Data Release 2. A990 was observed with LOFAR, as Observational evidence shows that the generation of radio ha- part of LoTSS, during Cycle 4 and is covered by three pointings: los depends on the dynamical state of the host clusters and their P154+50, P156+47 and P158+50. Details of the observations are mass (e.g. Wen & Han 2013; Cuciti et al. 2015). Radio halos are given in Table1. more likely to be observed in clusters that are massive and highly The LOFAR data was calibrated in two steps to correct disturbed. for the direction-independent and direction-dependent effects us- The power of radio halos is known to roughly correlate with ing Prefactor2 (van Weeren et al. 2016; Williams et al. 2016; their cluster mass and X-ray luminosities (e.g. Cassano et al. 2013; de Gasperin et al. 2019) and ddf − pipeline3(Tasse 2014a,b; Bîrzan et al. 2019). The scatters in the correlations reflect the evo- Smirnov & Tasse 2015; Tasse et al. 2018; Shimwell et al. 2019), lution of halos and dynamical state of clusters (e.g. Cuciti et al. respectively. We briefly outline the procedure below. For a full de- 2015). Spectral properties of the halos are also a contributing factor scription of the procedure, we refer to Shimwell et al.(2019). to the scatters. According to turbulent re-acceleration models these In the direction-independent calibration, the absolute ampli- properties reflect the efficiency of particle acceleration and are con- tude is corrected according to the Scaife & Heald(2012) flux nected with cluster mass and dynamics. Steep spectrum halos are scale. The calibration solutions are derived from the observations of generally expected in less massive systems and at higher redshift, 3C 196 which model has a total flux density of 83.1 Jy at 150 MHz. and are thought to be less powerful sitting below the radio power The initial XX-YY phase and clock offsets for each station are de- - cluster mass correlation (e.g. Cassano et al. 2006, 2010b, 2013; rived from the amplitude calibrator and are transferred to the target Brunetti et al. 2008). data. The target data are flagged and corrected for ionospheric Fara- Our understanding of the formation of radio halos are mainly day rotation4. During the data processing, the radio frequency in- based on studies of galaxy clusters that are more massive than terference (RFI) is removed with AOFlagger (Offringa et al. 2012) 14 5 × 10 " . There have been only a handful of radio halos de- when needed. The data are then phase calibrated against a wide- 14 tected in clusters that are less massive than 5 × 10 " (Abell field sky model that is extracted from the GMRT 150 MHz All-sky 545, Giovannini et al. 1999; Bacchi et al. 2003; Abell 141 Duch- Radio Survey (TGSS-ADR1) catalogue (Intema et al. 2017). esne et al. 2017; Abell 2061, Rudnick & Lemmerman 2009; Abell The direction dependent calibration step mainly aims to correct 2811, Duchesne et al. 2017; Abell 3562, Venturi et al. 2000, 2003; for the directional phase distortions caused by the ionosphere and Giacintucci et al. 2005; PSZ1 G018.75+23.57, Bernardi et al. 2016; the errors in the primary beam model of the LOFAR High Band An- RXC J1825.3+3026, Botteon et al. 2019; Abell 2146, Hlavacek- tennas (HBA). The direction dependent corrections are solved for Larrondo et al. 2018; Hoang et al. 2019). This is due to the sen- each antenna in kMS5 (Tasse 2014a,b; Smirnov & Tasse 2015) im- sitivity limitation of previous radio observations that are unable to plemented in the ddf − pipeline. The flux densities of the LOFAR detect faint radio halos in low-mass clusters. As a consequence, detected sources are corrected with the bootstrapping technique us- radio halos in the regime of low-mass clusters are largely unex- ing the VLSSr and WENSS catalogues (Hardcastle et al. 2016). plored. Steep-spectrum nature of radio halos makes low-frequency Prior to the bootstrapping, the WENSS catalogue is scaled by a fac- telescopes such as LOw Frequency ARray (LOFAR; Haarlem et al. tor of 0.9 to be consistent with the flux density scale in LoTSS (i.e. 2013) ideal instruments for studying radio halos in the low-mass Scaife & Heald 2012). To improve the image quality, the pipeline regime. uses a “lucky imaging” technique that generates additional weights In this paper, we present our search for extended radio emission to the visibilities basing on the quality of the calibration solutions from the galaxy cluster Abell 990 (hereafter A990). With a mass of (Bonnassieux et al. 2018). For imaging, DDFacet imager (Tasse 14 "500 = ¹4.9 ± 0.3º × 10 " (Planck Collaboration et al. 2016), et al. 2018) is used to deconvolve the calibrated data on each facet. A990 is an excellent target to search for cluster-scale radio emission To improve the quality of the final images in the cluster region, in a mass range that is below the typical one where radio halos are the data are processed through “extraction” and self-calibration found. We make use of the LOFAR Two-metre Sky Survey (LoTSS; steps. In the extraction, all sources outside of the cluster region Shimwell et al. 2017, 2019) 120 − 168 MHz data.